1. Field of the Invention
The present invention relates to a spark plug to be used as a source for igniting a mixed gas in an internal combustion engine and an insulator to be incorporated in such a spark plug.
2. Description of the Related Art
The insulator for spark plug (hereinafter referred as xe2x80x9cinsulatorxe2x80x9d) constituting the spark plug for use in internal combustion engines such as automobile engine is normally formed by an alumina-based sintered body obtained by sintering an alumina (Al2O3)-based insulation material. This is because alumina ceramics are excellent in heat resistance, mechanical strength, dielectric strength, etc. In particular, the insulator for spark plug is liable to exposure to a heat of from about 500xc2x0 C. to 700xc2x0 C. developed by the combustion (about 2,000xc2x0 C. to 3,000xc2x0 C.) of a gas ignited by spark discharge in the combustion chamber of internal combustion engine. Thus, it is important that the insulator for spark plug is excellent in dielectric strength over a temperature range of from room temperature to the foregoing high temperature. Such an insulator (alumina-based sintered body) has heretofore been formed by, e.g., a three-component system comprising silicon oxide (SiO2) calcium oxide (CaO) and magnesium oxide (MgO) as a sintering aid for the purpose of lowering the required sintering temperature and improving the sinterability.
However, the insulator formed merely by the foregoing three-component system sintering aid is disadvantageous in that the three-component system sintering aid (mainly composed of Si component) is present as a low melting glass phase on boundaries of alumina crystal particles after sintering. Thus, when the insulator is exposed to a heat of around 700xc2x0 C., the heat effect causes the low-melting glass phase to soften, possibly resulting in the deterioration of dielectric strength of the insulation material. It can be therefore proposed to merely reduce the amount of such a sintering aid to be added during the formation of the insulator for the purpose of reducing the occurrence of low-melting glass phase. However, this approach is disadvantageous in that the densification of insulator cannot proceed. Even if the densification of insulator proceeds apparently, numeral pores remain in boundaries of alumina crystal particles, possibly causing the deterioration of dielectric strength of insulator.
For the purpose of densifying the insulator, JP-A-62-100474 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes that a raw material composition obtained by granulating a raw material powder comprising alumina powder and the foregoing three-component system sintering aid to a predetermined particle diameter be blended with the same raw material composition which has not been granulated to reduce the amount of residual pores present on boundaries of alumina-based sintered body. JP-A-62-143866 proposes that a raw material powder comprising two alumina powders having different particle diameters and the foregoing three-component system sintering aid be sintered to reduce the amount of residual pores present on boundaries of alumina-based sintered body.
For the purpose of improving the dielectric strength of glass phase present on boundaries of alumina crystal particles, JP-B-7-17436 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent applicationxe2x80x9d), for example, proposes that an alumina-based sintered body be formed by a sintering aid such as Y2O3, La2O3 and ZrO2 to reduce the amount of residual pores and raise the melting point of glass phase present on boundaries of alumina crystal particles. Further, Japanese Patent 2564842 proposes that an alumina powder as a main component be blended with an organic metal compound and an aluminum compound to prepare a raw material powder having Y4Al2O9 phase uniformly dispersed in uniform alumina crystal particles at triple point so that the dielectric strength of the resulting alumina-based sintered body can be improved.
In recent years, with the enhancement of output of internal combustion engines and the reduction of the size of engines, the inlet valve and exhaust value have occupied more in the combustion chamber and the size of the spark plug has been reduced. Thus, the insulator constituting the spark plug has been required to be thinner and hence have a higher dielectric strength. Under these circumstances, however, even an insulator formed by the alumina-based sintered body according to the foregoing various patents can hardly meet the requirements for dielectric strength at a temperature as high as around 700xc2x0 C. sufficiently. Accordingly, such an insulator can undergo dielectric breakdown.
An object of the present invention is to provide a spark plug comprising an insulator containing alumina as a main component, which is less liable to occurrence of dielectric breakdown due to the effect of residual pores or low-melting glass phase present on boundaries of alumina-based sintered body constituting the insulation material and exhibits a higher dielectric strength at a temperature as high as around 700xc2x0 C. than the conventional materials and an insulator for use in such a spark plug.
The insulator for spark plug according to the invention which has been worked out to solve the foregoing problems comprises an alumina-based sintered body comprising Al2O3 (alumina) as a main component and at least one component (hereinafter referred to as xe2x80x9cxcex2 componentxe2x80x9d) selected from the group consisting of Ca (calcium) component, Sr (strontium) component and Ba (barium) component, the alumina-based sintered body having at least partly particles including a compound comprising the xcex2 component and Al (aluminum) component at an Al to xcex2 molar ratio of from 4.5 to 6.7 as calculated in terms of oxides thereof and having a relative density of 90% or more.
It is most noteworthy in the invention that the alumina-based sintered body comprising alumina as a main component comprises at least partly particles of a compound comprising xcex2 component and Al component at a molar ratio (Al2O3/xcex2O) of from 4.5 to 6.7 as calculated in terms of oxides thereof.
Since it can be presumed that the foregoing compound comprising specific components at a specific molar ratio is a compound having a high melting point, an insulator for spark plug formed by an alumina-based sintered body with particles made of such a compound present thereon can be provided with an extremely excellent dielectric strength at a temperature as high as around 700xc2x0 C. as compared with conventional insulators comprising alumina as a main component. Examples of the foregoing compound having a molar ratio (Al2O3/xcex2O) of from 4.5 to 6.7 include BaAl9.2O14.8, (molar ratio: 4.6; xcex2 component: Ba component), and BaAl13.2O20.8, (molar ratio: 6.6; xcex2 component: Ba component). Alternatively, compounds other than hexaaluminate and analogy thereof may be used.
The term xe2x80x9cparticlesxe2x80x9d as used herein is meant to indicate particles other than alumina particles observed on cut area obtained by cutting the insulator. The presence of these particles can be easily confirmed by mirror-polishing the cut surface of the insulator, and then observing the cut surface under SEM. If necessary, the presence of these particles may be confirmed by observing under TEM. Subsequently, these particles can be subjected to EDS analysis to confirm that xcex2 component and Al component are present therein.
Subsequently, the presence of the xe2x80x9ccompoundxe2x80x9d contained in the foregoing particles can be confirmed by various measuring methods. By way of example, an insulator which has been confirmed for the presence of particles comprising xcex2 component and Al component by observation under SEM and EDS analysis can be crushed to give a powder which is then subjected to X-ray diffractometry to see if there occurs a spectrum corresponding to the compound having a molar ratio (Al2O3/xcex2O) of from 4.5 to 6.7. If there is a spectrum corresponding to such a compound, it can be judged that the compound is present. In this X-ray diffractometry, if xcex2 component is Ba component, extremely similar spectra may be given with respect to X-ray diffractometry chart of BaAl9.2O14.8 (molar ratio: 4.6), BaAl12O19 (molar ratio: 6.0) and BaAl13.2O20.8 (molar ratio: 6.6), occasionally making it impossible to judge which compound is present. However, even in the case where any of the foregoing compounds is present, an effect of improving the dielectric strength at a temperature as high as around 700xc2x0 C. can be exerted so far as the foregoing molar ratio (Al2O3/xcex2O) falls within the range of from 4.5 to 6.7. Methods other than X-ray diffractometry (e.g., EPMA analysis) may be used to confirm the presence of the foregoing compound. It should be noted that different measuring methods may give a difference in molar ratio even with the same insulator. However, any measuring method makes it possible to exert an effect of improving the dielectric strength at a temperature as high as around 700xc2x0 C. so far as the foregoing molar ratio (Al2O3/xcex2O) falls within the predetermined range.
The site at which such particles are present is not specifically limited. The particles are preferably present in the interior of the insulator, more preferably on particle-particle boundaries and/or triple point of alumina. Further, these particles don""t need to be uniformly present in the alumina-based sintered body. These particles can be present intensively on the site where desired dielectric strength is required to exert an effect of improving dielectric strength. The shape of these particles is not specifically limited.
It is presumed that when the foregoing molar ratio (Al2O3/xcex2O) falls below 4.5 or exceeds 6.7, the compound formed by these specific components can have structural defects and thus exhibits deteriorated dielectric strength at a temperature as high as around 700xc2x0 C., although the reason for this phenomenon is unknown.
Further, in accordance with the present invention, it is important that the insulator not only comprises particles made of a compound comprising xcex2 component and Al component at a molar ratio (Al2O3/xcex2O) of from 4.5 to 6.7 as calculated in terms of oxide thereof but also has a relative density of not less than 90%. When the relative density of the insulator falls below 90%, many residual pores into which an electric field can be easily concentrated are present in the insulator, possibly causing the deterioration of improvement of dielectric strength at a temperature as high as around 700xc2x0 C. The term xe2x80x9crelative densityxe2x80x9d as used herein is meant to indicate the percentage of the density of the sintered body measured by Archimedes"" method per the theoretical density of the sintered body. The term xe2x80x9ctheoretical densityxe2x80x9d as used herein is meant to indicate the density obtained by converting the content of the various elements contained in the sintered body to an oxide basis, and then subjecting the results to calculation according to mixing theory. The more the relative density is, the more dense is the sintered body and hence the less is the amount of residual pores, i.e., the higher is the dielectric strength.
As mentioned above, the insulator according to the invention exhibits an excellent dielectric strength at a temperature as high as around 700xc2x0 C. as compared with the conventional spark plug. Hence, when applied to small-sized spark plug requiring a thin insulator or when applied to spark plug for high output internal combustion engine which exhibits a high temperature in the combustion chamber, the insulator according to the invention can effectively prevent troubles such as dielectric breakdown (penetration of spark).
Referring to the insulator for spark plug of the invention, it is judged that particles comprising a compound contributing to the improvement of dielectric strength have been formed when the molar ratio (Al2O3/xcex2O) of xcex2 component and Al component as calculated in terms of oxide falls within the predetermined range as mentioned above. Thus, the content of Al component and xcex2 component in the alumina-based sintered body are not specifically limited themselves. In order to obtain a good dielectric strength at a temperature as high as around 700xc2x0 C., however, it is preferred that Al component and xcex2 component be incorporated in the alumina-based sintered body in an amount of from 80.0% to 99.8% by weight (more preferably from 91.0 to 99.7% by weight) and from 0.2 to 10% by weight, respectively, based on 100% by weight of the alumina-based sintered body.
In the insulator for spark plug of the invention, the compound contained in the foregoing particles is preferably xcex2Al12O19 phase. The xcex2Al12O19 phase can be confirmed when charts similar to JCPDS (Joint Committee on Powder Diffraction Standards) card Nos. 38-0470, 26-0976 and 26-0135 on X-ray diffraction spectrum are obtained. JPSD card Nos. 38-0470, 26-0976 and 26-0135 indicate CaAl12O19 phase, SrAl12O19 phase and BaAl12O19 phase, respectively.
The reason why the dielectric strength of the insulator is enhanced when particles containing xcex2Al12O19 crystal phase are present at least locally in the alumina-based sintered body is unknown. This xcex2Al12O19 crystal phase is an ideal crystal structure among so-called hexaaluminate crystal structures and thus exhibits a high melting point as compared with other crystal structures having defects, presumably enhancing the dielectric strength at a temperature as high as around 700xc2x0 C. Regardless of which the particles present at least locally in the insulator (alumina-based sintered body) are composed of xcex2Al12O19 phase alone or along with other crystal, an effect of improving the dielectric strength can be exerted.
The insulator for spark plug of the invention may also comprise a silicon (Si) component. In this case, the molar ratio of content of silicon component and the foregoing xcex2 component as calculated in terms of oxide preferably satisfies the relationship SiO2/(SiO2+xcex2O)xe2x89xa60.8.
The Si component can easily melt to form a liquid phase during sintering to act as a sintering aid for accelerating the densification of the insulator. Thus, the incorporation of the Si component makes it possible to effectively enhance the densification of the insulator.
The foregoing Si component acts as a sintering aid for acceleration densification as well as exists as a low-melting glass phase on particle-particle boundaries of alumina crystal. In the present invention, when the insulator has particles made of a compound comprising xcex2 component and Al component at a molar ratio (Al2O3/xcex2O) of from 4.5 to 6.7 as calculated in terms of oxide, an effect of improving dielectric strength can be effectively exerted. Thus, the presence of particles having the foregoing properties on particle-particle boundaries in the alumina-based sintered body makes it possible to raise the melting point of particle-particle boundaries as compared with low-melting glass phase alone. It is important to adjust the proportion of Si component according to the foregoing relationship. This is because the adjustment of the proportion of Si component according to the foregoing relationship makes it possible to effectively produce particles having the foregoing properties on particle-particle boundaries during sintering. As a result, an effect of improving the dielectric strength of the insulator at a temperature as high as around 700xc2x0 C. can be effectively exerted.
The spark plug of the invention comprises an axial center electrode, a metal shell provided around the center electrode in a radial direction, a ground electrode fixed to the metal shell at one end thereof opposed to the center electrode, and an insulator for spark plug as shown above provided around the center electrode in a radial direction interposed between the center electrode and the metal shell. In this arrangement, a spark plug can be formed having an insulator which exhibits an excellent dielectric strength at a temperature as high as around 700xc2x0 C. and can hardly undergo dielectric breakdown (penetration of spark).